What causes "Cannot add tasks to a queue that has ended" in sveltejs/kit?

Ensure all q.add() calls complete before invoking q.end() — await outstanding tasks first

Cannot add tasks to a queue that has ended

Error message

Cannot add tasks to a queue that has ended

What it means

SvelteKit's internal task queue (used during prerendering/build) exposes add() for scheduling work. Once q.end() is called the queue is closed; adding more tasks afterwards throws this error. It indicates scheduling work after the build phase has been finalized.

Source

Thrown at packages/kit/src/core/postbuild/queue.js:52

					.then(task.fulfil, (err) => {
						task.reject(err);
						reject(err);
					})
					.then(() => {
						current -= 1;
						dequeue();
					});
			} else if (current === 0) {
				closed = true;
				resolve();
			}
		}
	}

	return {
		/** @param {() => any} fn */
		add: (fn) => {
			if (closed) throw new Error('Cannot add tasks to a queue that has ended');

			const promise = new Promise((fulfil, reject) => {
				tasks.push({ fn, fulfil, reject });
			});
			promise.catch(() => {});

			dequeue();
			return promise;
		},

		done: () => {
			if (current === 0) {
				closed = true;
				resolve();
			}

			return promise;
		}

View on GitHub (pinned to 03f1687fe6)

Solutions

Ensure all q.add() calls complete before invoking q.end() — await outstanding tasks first
Reorder code so end() is only called after every producer has finished enqueueing
Track closed state in your wrapper and skip/queue-later instead of calling add after end
If triggered by a plugin, update the plugin to a version compatible with your @sveltejs/kit build pipeline

Example fix

// before
await Promise.all(items.map((i) => q.add(() => render(i))));
q.end();
stragglers.forEach((i) => q.add(() => render(i))); // throws

// after
for (const i of items) await q.add(() => render(i));
for (const i of stragglers) await q.add(() => render(i));
q.end();

Defensive patterns

Strategy: try-catch

Validate before calling

function safeAdd(q, closed, fn) {
  if (closed) return Promise.resolve();
  return q.add(fn);
}

Try / catch

try {
  await q.add(task);
} catch (e) {
  if (String(e.message).includes('queue that has ended')) {
    console.warn('Queue closed; run task directly or reschedule');
    return task();
  }
  throw e;
}

Prevention

Await every q.add() promise before calling q.end()
Never enqueue from callbacks that can resolve after teardown
Keep one owner of the queue lifecycle in your build code

When it happens

Trigger: Calling q.add(fn) after q.end() on the queue returned by create_queue — in practice, code or a plugin hook that enqueues prerender/render tasks after the prerender step completes.

Common situations: Custom build tooling integrating with Kit internals; async callbacks resolving after q.end() and then trying to add work; misordered lifecycle code in postbuild scripts.

Related errors

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/5926a814924eb314. Report an issue: GitHub.